B7-H1 and B7-H4 are two B7 family molecules with potent immunosuppressive functions. Upon interaction with their respective counter-receptors on T cells, these B7Hs constitute essential immunologic checkpoints in tuning down T cell responses to prevent overly activated and harmful immune responses during infection and inflammation. The long-term goal of this study is to elucidate the mechanisms of B7-H1 and B7-H4 in the induction of tolerance in tumor microenvironment as a means to design novel approaches to enhance cancer immunity. Recent studies demonstrate that B7-H1 is broadly up-regulated on cancer cells, tumor-infiltrating lymphocytes and other stromal cells. In addition, expression of B7-H1 is found to correlate with poor prognosis of renal cell carcinoma and esophageal cancer. Inhibitory effect of B7-H1 on T cells is at least partially mediated through binding to receptor PD-1. B7-H4 is also upregulated in various types of cancer cells. In addition, B7-H4 is found to express on tumor-infiltrating macrophages in high level and renders their suppressive function. The central hypothesis of this proposal is that B7-H1 and B7-H4 in tumor microenvironment induces tolerance so as to prevent immune destruction. Using mice with spontaneous prostate cancer as a model, we will use B7-H1/B7-H4 specific neutralizing monoclonal antibodies as a general approach to determine the role of these molecules in T cell tolerance and cancer progression. In addition, B7-H1/B7-H4 deficient mice will be employed to selectively ablate the expression of these molecules in subsets of host and immune cells to dissect the mechanisms of cancer microenvironment B7- H1 and B7-H4 in the induction of immune tolerance. The contribution of PD-1 on T cells and stromal cells in cancer microenvironment will also be evaluated. By the completion of these studies, we will gain insight into tolerance mechanisms of B7-Hs in cancer microenvironment and, more importantly, provide the basis for future design of immunotherapy.